The combo cell renderer takes care of adding a text cell renderer to the
combo box and sets it to display the column specified by its
cellTextModel property. Further cell renderers can be added in a handler
for the editingStarted signal.

The tuple containing a model and a column in this model that determine
the possible strings that can be shown in the combo box. Note that this
tree model can be a datum in the tree model that is used to populate the
view in which the CellRendererCombo is part of. In other words, it is
possible that every CellRendererCombo can show a different set of
options on each row.